Mechanical Basics

What Is Diesel Cycle?

Diesel Cycle is a type of Reversible ideal internal combustion engine like the Otto Cycle that uses Diesel as fuel for its working. Diesel Cycle can be called Diesel Engine Cycle and Constant Pressure Heat Addition CycleDiesel Cycle is invented by Rudolf Diesel in the 1890s and stood competitor for gas engines.

Similar to Otto Cycle, the Diesel Cycle does consists of four processes and four strokes in its construction. As the Diesel Cycle observes and rejects the heat for its working, it is also called a Thermodynamic Air Cycle

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What can you learn from this post?

  • Diesel cycle process
  • Efficiency of Diesel Cycle
  • Limitations of Diesel Cycle
  • Conclusion
  • Frequently Asked Questions

Diesel cycle process:

Diesel Cycle (Internal Combustion Engine), Diesel Cycle Process, And Its Efficiency

The Diesel cycle uses diesel as its working fuel and it will not use any spark igniters like Otto Cycle. In this cycle, the air is compressed at high temperature whereas; the diesel will sprinkle at the end of the compression process which results in the combustion process. 

There are Four Processes in the Diesel Engine which result in the completion of one cycle. As this process is reversible, the four processes will again work with the same principle and structure. The four processes of the Diesel Engine Cycle are.

Reversible Isentropic Compression Process (1-2):

Process 1-2 is the Reversible Isentropic Compression Process also called Adiabatic Compression Process. In this Diesel Cycle Process, the entropy will be constant (which is called as Isentropic Process) and no heat is added or rejected (which is called as Adiabatic Process)

Due to the pressure increase, the piston in the cylinder will move from the Bottom Dead Center (BDC) to the Top Dead Center (TDC). The volume of the internal combustion engine cylinder will be decreased highly due to the piston movement (observe P-V diagram). 

Diesel Cycle (Internal Combustion Engine), Diesel Cycle Process, And Its Efficiency

This process is similar to Compression Process in the real internal combustion engine.

Constant Pressure Heat Addition Process (2-3):

Process 2-3 is called as Constant Volume Heat Addition Process. In this Diesel Cycle Process, the Heat is added at constant pressure so, the entropy and temperature will be increased.

Due to the Constant Pressure Heat Addition, the pressure remains unchanged and the piston in the internal combustion engine cylinder stays at Top Dead Center (TDC) without any movement.

The Constant Pressure Heat Addition will be similar  to the spark ignition and combustion process in the real internal combustion engine.

Reversible Isentropic Expansion Process (3-4):

Process 3-4 is the Reversible Isentropic Expansion Process also called the Adiabatic Expansion Process. In this Diesel Cycle Process, the entropy will be constant (which is called as Isentropic Process) and no heat is added or rejected (which is called as Adiabatic Process)

Due to the slight pressure, the piston in the cylinder will move from Top Dead Center (TDC) to the Bottom Dead Center (BDC). The volume of the internal combustion engine cylinder will be increased due to the piston movement (observe P-V diagram).

This process will be the same as the Expansion Process in the real internal combustion engine.

Constant Volume Heat Rejection Process (4-1):

process 4-1 is the Constant Volume Heat Rejection Process. In this Cycle Process, the Heat is rejected at a constant volume so, the entropy and temperature will be decreased.

Due to the Heat Rejection, the pressure decreases highly and the piston in the internal combustion engine cylinder stays at the Bottom Dead Center (TDC) without any movement.

This process will be the same as Out-take Process in the real internal combustion engine.

Efficiency of Diesel Cycle:

Efficiency is a standard calculating term that states the performance of an internal combustion engine or cycle. The efficiency of the Diesel Engine is higher than the Petrol or Otto Cycle; in this cycle, we use a fuel injector instead of a spark igniter for the combustion process.

The efficiency of the Diesel Cycle is defined as the ratio of Work Done to the Heat Absorbed or added. So, the efficiency of Diesel Cycle mathematical expression will be as follows.

Diesel Cycle (Internal Combustion Engine), Diesel Cycle Process, And Its Efficiency

According to the constant pressure heat addition process and constant volume heat rejection, the heat absorbed and rejected can be taken as

Diesel Cycle (Internal Combustion Engine), Diesel Cycle Process, And Its Efficiency

Now, substituting the above formula in the equation 1, then

Diesel Cycle (Internal Combustion Engine), Diesel Cycle Process, And Its Efficiency

According to the isentropic compression and isentropic expansion processes, the temperatures and volume can be related as

Otto Cycle (Internal Combustion Engine), Otto Cycle Process, And Its Efficiency

 where r is the compression ratio. Then the Efficiency of Diesel Cycle is

Diesel Cycle (Internal Combustion Engine), Diesel Cycle Process, And Its Efficiency

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Limitations of Diesel Cycle:

The limitations of Diesel Cycle are 

  • The Diesel Cycle is limited to the Diesel internal combustion engines only and does not work out for petrol and gas engines.
  • The efficiency of Diesel Cycle is limited and not able to achieve as much as Carnot Cycle does.

Conclusion:

The Diesel Engine Cycle is an ideal cycle for real-life diesel engines. We can observe many diesel engine automobiles, generators, etc. In the Otto Cycle, the spark igniter is used to perform the combustion process but due to the self-igniting property of the diesel engine, any spark igniters are used.

The diesel engines are used in heavy trucks that transport heavy loads from one place to another place. Diesel Engine Cycle is even used in the generators.

Frequently Asked Questions

The Diesel Engine works with the fuel Diesel. In the Diesel Engine, the fuel injector is used for combustion process and no other fire igniters are required. Apart from this, the diesel engine consists of four processes which we call as Compression, Expansion, In-take, Combustion, and Exhaust. So these 5 processes are the reason of how does a diesel engine work.

The four cycles in Diesel Engine are

  • Isentropic Compression Process
  • Constant Pressure Heat addition process
  • Isentropic expansion process
  • Constant volume heat rejection process

The two types of diesel engine are

  • Four Stroke Engine
  • Two Stroke Engine

The two types of diesel engine are

  • Four Stroke Engine
  • Two Stroke Engine

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